1. Field of the Invention
The present invention relates to a method and device for waveform shaping of signal light.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional waveform shaping device for performing waveform shaping at the optical level. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs—SOA having opposite end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 μm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as a SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing at the optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
The 2R functions can be provided by combining a waveform shaper and an optical amplifier, or by using a waveform shaper having an optical amplifying function. Further, the 3R functions can be provided by additionally using a clock regenerator in parallel to the 2R functions.
By combining an opto-electrical converter, a discriminating circuit, and an electro-optical converter, it is possible to provide a regenerative repeater for performing waveform shaping in the electrical domain. In this kind of regenerative repeater, a threshold can be changed by changing a set value of a reference voltage in the discriminating circuit.
However, in an all-optical regenerative repeater, it is not easy to change a threshold. For example, in the above-mentioned NOLM, a transfer function is determined according to a fiber length and a nonlinear coefficient, so that when an optical fiber is given as the nonlinear medium, the threshold in its input-output characteristic is determined uniquely and it cannot be changed. Accordingly, the threshold is not always an optimum threshold for signal light supplied, and there is a possibility that the regenerating operation may become incomplete.